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If there aren't enough arguments, we print to the standard error stream to inform the user on how to use the program. This has 4 arguments so we need to check if the user has entered less than what we need by comparing argc. We want to have our program parse the command line like so: We've planned our pseudocode and have the necessary functions so let's begin making our basic prototype. If we are on a Windows machine, we simply require the output file to have the. Since we require execution permissions we'll use the S_IXUSR flag (keep in mind that this will make the permissions of the file only executable). We also need a mode which are detailed in the man page. Like execve, we require a pointer to a string of our target file's name. In C, we have a function which can do the same job and guess what it's called. In the command line, we can do this with the chmod utility. Because of this, we may require a function to programmatically and automatically do this for us. On a linux machine, when we output a file, it may not have the necessary permissions to be executed. Overall, the execve requires a char * array which has pointers to the name of our target executable file as a string and environment arguments (which we do not need for this so it will just be NULL). Head over to the manpage for execve for complete details. Fear not, for I have found a suitable one for us. We have yet to discover a function which will execute a file (I do not recommend using the system function). If you have completed the task I assigned in the previous tutorial the hard part is done. So our deobfuscation routine will be exactly the same! How wonderfully convenient!

Write the obfuscated contents into the output file.Īs for the deobfuscation part, remember that XOR can decrypt the encrypted as long as we use the same key.

Extract the contents one-by-one from our malware.

Open an output file to disk with a binary write.

Open our malware from disk with a binary read.

Let's now create a plan on how to code our crypter. We've completed our checklist and understand how crypters work. Obviously, we do not have the knowledge to create the runtime crypter because we are not familiar with either the Windows API or how executables are loaded into memory, so we will have to settle with a scantime.

File I/O to read our malware and write to an obfuscated malware.

We will need to gather what we know before we can start planning the code. So now that we know how crypters work, we can attempt to create one. There is a flag CREATE_SUSPENDED which allows the malware to be decrypted and then loaded into memory as a process before being executed. Runtime crypters are able to do this with the Windows API using a function called CreateProcess.

Scantime crypters are only capable of decrypting the malware in disk before executing it. The runtime crypter is a scantime crypter with the ability to decrypt the malware while it is running.

As their name suggests, scantime crypters only evade antivirus detected as an idle file on disk.

There are two types of crypters: scantime and runtime. Once the stub is opened by the victim, it will decrypt the malware and execute it. It then outputs a seemingly "harmless" file called the stub. How it does this is it applies an obfuscation method onto the malware so that the antivirus cannot successfully match it with any signature. What Is a Crypter?Ī crypter is a program which is used to assist malware with evading antivirus signature-based detection. After defining these, we will then make our own basic crypter using the function I gave to you as a task to complete by yourself in the previous tutorial. In this article, we will be discussing types of crypters, how they work and why they work. Welcome to a tutorial on crypting technology.